less
vital than is that of physiological chemistry to studies of animal life.

The common conception that plant life and animal life are antithetical or
complementary to each other has much to justify it. Animals breathe in
oxygen and exhale carbon dioxide; while plants use the carbon dioxide of
the air as a part of the raw material for photosynthesis and exhale oxygen.
Plants absorb simple gases and mineral compounds as raw food materials and
build these up into complex carbohydrates, proteins, fats, etc.; while
animals use these complex compounds of plant origin as food, transforming
parts of them into various other forms of structural material, but in the
end breaking them down again into the simple gases and mineral compounds,
which are expelled from the body through the excretory organs. Thus it
would seem that the study of the chemistry of plant life and of animal life
must necessarily deal with opposite types of phenomena.

But one cannot advance far into the study of the biochemistry of plants and
animals before he discovers marked similarities in the chemical principles
involved. Many of the compounds are identical in structure, undergo similar
changes, and are acted upon by similar catalysts. Plant cells exhibit
respiratory activities, using oxygen and giving off carbon dioxide, in
exactly the same way that animal organisms do. The constructive
photosynthetic processes of green plants are regulated and controlled by a
pigment, chlorophyll, which is almost identical with the blood pigment,
haematin, which regulates the vital activities in the animal organism,
differing from the latter only in the mineral element which links the
characteristic structural units together in the molecule. Many other points
of similarity in the chemistry of the life processes of plants and animals
will become apparent as the study progresses. It is sufficient now to call
attention to the fact that these vital processes, in either plants or
animals, are essentially chemical in character, and subject to study by the
usual methods of biochemical investigations.

The protoplasm of the cell is the laboratory in which all the changes which
constitute the vital activities of the plant take place. All of the
processes which constitute these activities--assimilation, translocation,
metabolism, and respiration--involve definite chemical changes. In so far
as it is possible to study each of these activities independently of the
others, they